A downhole apparatus may have contained therein a fluid chamber for a variety of purposes. For example, a compounder used in tandem with a jar may use a fluid chamber as an inner spring mechanism in order to store additional energy used to enhance a jar. U.S. Pat. No. 5,931,242 describes a double-acting compounder that incorporates a movable piston disposed within a fluid chamber between inner and outer cylindrical assemblies to provide compounding in either jarring direction.